What do you look for most when buying Multifamily?

[Maddan Pun]

I know about cap rate, NOI and other metrics. But what is the most important thing to look for when buying your Multifamily (4+ units)? For example, it is a slightly older apartment with couple units needing full rehab. Is there a multifamily calculator? I have been using BP calculator for SFH.

Thanks,

Maddan